“Fake news is not simply a political abstraction. It’s a real offense that harms real people.”

From a New York Times review by Jamie Poniewozik headlined Review: ‘After truth,’ the Deluge: An often-chilling survey of disinformation in America, this HBO documentary reclaims the real definition of “fake news.”

It is a fitting irony that the term “fake news” has become itself fraudulent, appropriated, by Donald Trump and his imitators, to dismiss legitimate reporting that they deem damaging, disrespectful or insufficiently flattering.

But before there was this fake news, there was real fake news, an ecosystem of rumors, conspiracy theories, frauds and hoax stories, some of which were deployed in 2016 to boost President Trump’s campaign. It’s this modern weapon that is the subject of HBO’s “After Truth: Disinformation and the Cost of Fake News,” a broad but darkly engrossing documentary that airs Thursday.

“After Truth,” directed by Andrew Rossi and executive produced by CNN’s Brian Stelter, makes clear that fake news is not something invented in 2016 or limited to partisan politics. It is abetted by modern information technology, but the root of this evil is opportunism and cynicism. . . .

All this mistrust, of course, has its own origins and antecedents. The paranoia about President Obama didn’t come from nowhere; he was the subject of hoaxes and frauds from early on (including the big one, birtherism, espoused by his successor). And decades of mistrust-sowing by partisan media and politicians helped create the free-for-all in which everyone feels entitled to bespoke facts.

“After Truth” doesn’t do much historical delving. Instead it presents a reel of fever-swamp highlights: the Pizzagate fiction that convinced a gunman that a Washington, D.C., restaurant was a front for an elite pedophilia ring; the wild theories about the attempted-robbery murder of Seth Rich, a Democratic National Committee staffer, that Fox News’s Sean Hannity amplified; the adoption of fake-news tactics by activists on the left in the 2017 Alabama senate election; and the resistance of social-media companies, especially Facebook, to do anything to halt the spread. . . .

In other places “After Truth” plays more like a survey, something that might have made an hourlong feature on CNN. But its strongest animating idea is that fake news is not simply a political abstraction. It’s a real offense that harms real people, from the parents of school-shooting victims maligned by Infowars to the threatened staff and customers of a pizza place.